Association of the 3,5,3'-triiodo-L-thyronine nuclear receptor with the nuclear matrix of cultured growth hormone-producing rat pituitary tumor cells (GC cells)

The iodothyronine nuclear receptor, a nonhistone chromatin peptide, mediates growth hormone gene transcription in cultured GC cells (Yaffe, B.M., and Samuels, H.H. (1984) J. Biol. Chem. 259, 6284-6291). To determine whether the 3,5,3'-triiodo-L-thyronine (T₃) receptor was localized to the nuclear matrix, we studied the subnuclear distribution of [¹²⁵I]T₃-receptor complexes after treatment of nuclei with DNaseI and 2 M NaCl to facilitate removal of histones. After incubation with 5 nM [¹²⁵I]T₃ to exchange with 80-90% of nuclear T₃ receptors, the nuclear matrix fraction contained < 1% of nuclear DNA, 16.5% of nuclear protein, and 30-50% (mean, 40.0 ± 2.3%) of specifically bound [¹²⁵I]T₃. Control experiments showed that nuclear matrix [¹²⁵]T₃ did not appear exchangeable with added 5 nM T₃ and did not result from release and nonspecific precipitation of [¹²⁵I]T₃ or [¹²⁵I]T₃-receptor complexes during nuclear matrix preparation. Studies with the T₃ photoaffinity probe, N-2-diazo-3,3,3-trifluoropropionyl-[¹²⁵I]T₃ resulted in the finding of limited capacity receptor forms, 58,000 and 46,000 kDa, in the nuclear matrix. These receptor forms were identical to those observed when N-2-diazo-3,3,3-trifluoropropionyl-[¹²⁵I]T₃-labeled receptors were solubilized directly from nuclei. Lastly, limited capacity [¹²⁵I]T₃ binding was demonstrated in 0.4 M KCl buffer extracts of nuclear matrix. Binding displacement studies suggested that 46% of the binding activity solubilized from nuclear matrix exchanged with [¹²⁵I]T₃ and that the apparent equilibrium association constant of this binding activity was similar to that of 0.4 M KCl extracts of isolated nuclei. These results suggest that an appreciable fraction of nuclear T₃ receptors is localized to the nuclear matrix and may influence the expression of thyroid hormone action.